Process of producing corrosion-resisting coating on iron and steel and products



NOV- 29,'1927o UNITED STATES PATENT OFFICE;

CHAD H. HUMPHBIES, OI KOKOMO, INDIANA, ASSIGNOB, BY MESNE ASSIGNMENTS, TO

METALS PROTECTION CORPORATION, OF INDIANAPOLIS, INDIANA, A CORPORA- TION O1 INDIANA.

P300358 01' PRODUCING COBBOBION-BESIBTING COATING ON IRON AND STEEL AND PRODUCTS.

Drawing.

I have found that by applying a very thin coating of chromium over the coatings of other metals upon iron and steel products having very desirable corrosion resisting 5 properties may be produced, and further that the chromium coating can be produced in such form that further mechanical finishing thereof is unnecessary.

In a copending application in the names 10, of Harrie C. Pierce and Chad H. Humphries, Serial No. 711,256,filed May 25, 1924, (now Patent No. 1,545,196) there is described a method of producing extremely thin continuous and coherent coatings of chromium upon other metals. Such chromium coatings have excellent corrosion preventing properties. Under certain circumstances, however, it is unwise to trust solely to the chromium coating. The present invention therefore relates to the application of multiple coatings of certain metals, the outermost of which coatings is chromium, and to the products so produced.

The invention embraces quite a variety of combinations of coatings among which may be mentioned by way of example the fol lowing: chromium on copper on iron or steel; chromium on brass on iron or steel; chromium on copper on cadmium on iron or steel; chromium on copper on zinc on iron or steel; chromium on brass on cadmium on iron or steel; chromium on brass on zinc on iron or steel; chromium on cadmium on iron or steel; chromium on zinc on iron or steel;

chromium on nickel on iron or steel.

7 The coatings may be applied by any suitable electroplating methods but with re ard to the chromium coating it is desirable cm a commercial standpoint that a process be used which will give a lustrous coating at low current density, otherwise, for instance if a high current density is required or if the coating isnot lustrous, some of'the value of the composite coating from a commercial standpoint is lost. I therefore prefer to use the process of forms the subject matter of the Pierce and Humphries application Serial No. 711,256, above referred to. Said process consists to the chromium sulfate.

coating described above.

chromium plating which Application fled June 5, 1924. Se11a1 No. 718,080.

generally in electrodepositing.chromium at a relatlvely low current density, say from 14 up to. 125 amperes per square foot, but

preferably in the neighborhood of 14 to 40 amperes per square foot, from a bath conta1n1ng essentially chromic acid, ammonium sulfate and hydrated chromic hydroxid dissolved 1n water at room or slightly elevated temperature, using either inert or chromium anodes.

The bath may be prepared by mixing a solut on of chromic acid in water with a solution of the reaction products of chromic sulfate and ammonium hydroxide in water. The quantity of chromic acid used may vary from 80 to 450 grams per liter of the final solution. The chromic sulfate ,is used in quantity amounting to 1 to 2% of the chromic acid and the ammonium hydroxide preferably 1n quantity about chemically equivalent 0 The invention is not limlted to the use of the particular bath and process for applying the chromium The SOY-called Sargents solution for which somewhat hlgher current densities are recommended may be used. 7

The following examples illustrate the process:

(1) Iron or steel articles, properly prepared mechanically for finish and by ordinary plating room cleaning methods to receive an electrodeposited coating, are plated in a zinc cyanide bath with a coating of zinc approximately .0001 inch or more in thickness, rlnsed in water and given an electro- 86 deposited coating of copperor brass from a copper cyanide or standard brass plating bath, subjected to mechanical bufling or to a bright acid dip to give the copper or brass coating a high luster, and then coated with 9 chromium by means of the Pierce and Humphries chromium plating process described above.

(2) Iron or steel articles properly prepared for electroplating and finishing as described in Example 1 are plated with copper in a cyanide copper plating bath alone or in a cyanide copper bath and then in an uacid copper bath to produce the desired copper coating .0001 inch or less thick and" the chromium coating .00006 inch or less thick by computation, after being sub ected to the standard salt s ray test for ours showed no evidence of rust streaks or areas, or pin holes, and the original brightluster of the chromium. coating was undlmmed. For comparison it is noted that standard nickel-on steel and nickel on copper on steel platings subjected to the same salt s ray test failed in 10 hours, while a nicke on cadmium on steel coating failed in 24 hours. The chromium coated articles not only withstand corrosion longer than the other standard coatings referred to, but have the additional and very important further advantage that the retain their luster and do not be come du led or tarnished.

The chromium coatings should not be of less than a com uted thickness of .0O0Q6 inch, because wit coatings of less than the thickness there is a tendency for the under coatin to show through after standing, there ing evidence of chromium absorption.

Of the composite coatings those in which the chromium over-coating is deposited directly upon an under-coatm of copper or brass are preferred because c romium coatings upon copper and brass show a ater tendency to e lustrous and the pro uction of a lustrous finish directl obviates the more or less expensive mechanical coloring treatment which otherwise would be necessary in order to produce lustrous finishes. When the copper or brass coating is deposited directly upon the iron or steel it s ould be made somewhat heavier than when the undercoatin of cadmium or zinc is used.

e corrosion resisting properties of the composite coatings of the present invention may be explained in part by the fact that the chromium coatings are very hard and protect the undercoatingagainst abrasion. A .further explanation is as follows.

When iron is protected against corrosion by coatin of metals such as cadmium and zinc whi are electropositive to iron the protection is given by the preferential oxidation of the coating metal while when coatings of nickel alone or nickel on copper on iron are used, the protective action is due solely to the continuity of the coatings since both co per and nicke are electronegative to iron. the case of coatings of mckel on v chromium. B

cadmium or zinc on iron the intermediate metal, cadmium or zinc which is electropositive to both the iron and nickel, is ver rapidly used up due to electrolytic action in case in holes or breaks in either the iron or nic el occur. Such conditions do not exist in the coatings of the present invention. For instance in the case of the coatin of chromium on co per on cadmium on iron the relationship 1s as follows. Cadmium is positive to iron, copper is ne tive to the cadmium and the chromium is e ectropositive to both the copper with which it is in contact and also to the iron. In the case of chromium on co per on iron, the more electropositive meta is on the outside and not as 1n the case of nickel on cadmium on iron, sandwiched between two electrons tive metals. In the case of chromium on me on iron both of the coating metals are electropositive to the iron.

Throughout the specification and in the claims, I mean by an electropositive metal, a metal electropositive to iron, and above iron in the electromotive series, such as zinc, or electronegative metals, I mean metals be ow iron in the electromotive series, such metals being nickel co per, etc.

There is a discrepancy in t e iterature with regard to the designation of elements in the electromotive series and for purposes of clarity and consistenc the followin table, taken from Mellors Modern rganic Chemistry (1917 edition, page 363) isappended.

Sodium. Cobalt. Magnesium. Nickel. Chromium. Tin. Manganese. Lead.

' Zinc- Hydrogen.

Cadmium. Copper, Iron.

The more electro ositive metals are at the top of the table. T us zinc is electropositive to iron whereas copper or nickel is electrone tive to iron.

ot all of the elements listed in the Mellor table have been listed herein. The-p of the table is to show the and neglativity of the metals towards iron. Thus, c romium on copper on cadmium on iron or steel bears the relation tentially speaking, of a metal electropositive to iron superposed on an electronegative metal which is in turn an rposed on an electropositive metal plated I claim:

I ow y relative positivity on an iron or steel base.

1. Process of protecting iron and steel 1,661,278 I Y b prising a layer of copper and. a superposed to be' hated, then electroplating a'copper layer of chromium. o coat ercon, and finally electrop1etin'g' a 10 3. An electroplated article consisting of chromium'coat on the copper coat. p f a steel base, a copper coat electro-deposited 5. The method of protecting steel which I 6 thereon, and a chromium coat electro-deconsists in coating it with copper endich'ro posited on the copper.

4. A process 0 electro lating steelor the Intestimony whereof, I aflix my signature. I like, consisting in first c caning the surface I CHAD H. HUMPHRIES. 4 

